Polishing silicon wafers

ABSTRACT

An endless belt for a belt type polishing machine comprises a support fabric and a polymer layer of relatively low hardness. The polymer layer is formed with drainage grooves. The support fabric may comprise a non woven or woven material, or a membrane with oriented reinforcing yarns. A further version comprises a spiral-link fabric supporting a woven or non woven layer carrying the polymer layer. The polymer layer may be a double layer, the upper of which is either harder or softer than the lower layer.

BACKGROUND OF THE INVENTION

[0001] Silicon wafers are produced as precursors from whichmicro-electronic semiconductor components are produced. The wafers aregrown for example by deposition of silicon onto a substrate, to producediscs typically 20 cm in diameter, which are split by cleavage parallelto their major surfaces (analogous to the cleavage of slate) to producetwo thinner wafers. The resulting wafers require to be polished to givetotally flat and planar surfaces for deposition of electronic componentsonto the surface by standard lithographic and etching techniques to formintegrated chip semiconductor devices. Typically a 20 cm diameter waferwill produce forty micro processor chips.

[0002] The designed size of such integrated chips is steadily decreasingand the number of layers applied, e.g. by lithography onto the siliconsurface is rising, to produce ever smaller and increasingly complexmicro-circuits. Present semiconductors typically incorporate 3 or 4metal layers, whilst it is expected that future designs will contain 5or more layers. This increase in the number of layers applied is leadingto ever more stringent requirements on the smoothness and planarity ofthe silicon wafers, since pits or scratches may produce voids whichcannot be bridged by deposited material, as the widths and thicknessesof deposited layers are decreased, leading to unplanned resistanceswhere a conductor is narrowed, or capacitances/non-conductive gaps,where breaks occur in deposited conductor layers, which interfere withor compromise the planned operation of the circuit.

[0003] The standard wafer polishing technique in use at present is toremove a wafer from a stack, or cassette of e.g. 10 wafers, by means ofa robot arm, and manoeuvre the disc into position over a rotating disc.The disc is usually coated with polyurethane, and the wafer is held inplace by an overhead platen whilst being polished by the rotating disc.This is an adaptation of optical polishing technology used for polishinglenses, mirrors and other optical components. Once polishing iscompleted, the robot arm removes the wafer and transfers it to anotherwork station for eventual lithographic deposition steps.

[0004] A significantly different approach is so-called LinearPlanarisation Technology, developed by OnTrak, wherein an endlesstravelling belt is used to polish the wafer, in place of the rotatingdisc form of polishing tool. The belt used in this method is describedin EP-A-0696495 and comprises an endless belt of sheet steel, having apolyurethane coating of low Shore A hardness. A major problem with thesebelts is the poor adhesion of polyurethane to steel. An adhesive orcoupling agent is required for bonding between the steel andpolyurethane to take place but in spite of the use of such an agent bondstrength is insufficient to withstand the harsh conditions under whichthe belt operates—particularly the frictional forces occurring betweenthe belt and wafer in the zone of contact. The tendency is for thepolyurethane to wear out or to flake off within two days or so, and torepair this an area around the damaged coating has to be removed forfresh polyurethane to be added as a patch. This leaves seams or jointsbetween the original coating and the patches which must be removed bycomplicated and expensive high-precision machinery and processes so asto ensure that a flat planar belt surface is maintained.

OBJECTS OF THE INVENTION

[0005] An object of the invention is to provide a belt-type apparatusfor polishing silicon wafers wherein the problems arising from the useof a sheet metal belt, having a poorly bounded coating, are at leastsubstantially overcome.

SUMMARY OF THE INVENTION

[0006] This invention provides for use in polishing silicon wafers, anendless belt to act as a polishing tool, characterised in that the beltcomprises a woven or non-woven fabric coated with a suitable polymer.

[0007] The polymer is preferably polyurethane, preferably with a lowShore D hardness, e.g. from 65-75.

[0008] Alternatively the polymer may be any thermoset or thermoplasticpolymer having a reasonably high abrasion resistance, such aspolyamides, silicones, fluoropolymers, epoxy resins and thermoplasticpolyurethanes.

[0009] The coating may comprise two or more layers of differenthardnesses. The coating may comprise at least one layer of partiallyfused polymeric particles, or two or more thermoplastic polymers ofdifferent melting points.

[0010] The upper layer may be the harder layer.

[0011] On the other hand the intermediate layer may be the harder layer,and the upper layer may comprise a foamed plastic, or be formed of orincorporate thermally expandable expanded polystyrene beads which formpores in the plastics layer. Hollow microbeads of plastics, glass orsoluble material may be incorporated in the upper layer.

[0012] Abrasive particles or fibres may be added to the upper layer,which may constitute a transparent coating, or be micro textured withmicro-scale grooves or surface roughness.

[0013] The fabric may be a substrate which is woven in endless formembodying yarns of high tensile strength and relatively low elongation.

[0014] A fabric woven in endless form lacks the weak spots of a seam orsplice, which is a great advantage as these belts operate underextremely high tension to prevent the formation of ripples or wrinkles.

[0015] The belt thickness is typically 01.-0.2 inches, whilst thecoating thickness is in the range 0.005-0.09 inches.

[0016] Examples of suitable yarns are meta- or para- aramids such asKEVLAR, NOMEX or TWARON; PBO or its derivatives; polyetherimide;polyimide; polyetherketone; PEEK; gel-spun UHMW polyethylene (such asDYNEEMA or SPECTRA); or polymenzimidazole; or other yarns commonly usedin high-performance fabrics such as those for making aerospace parts.Mixtures or blends of any two or more yarns may be used, as may glassfibres (preferably sized), carbon or ceramic yarns including basalt orother rock fibres, or mixtures of such mineral fibres with syntheticpolymer yarns. Any of the above yarns may be blended with organic yarnssuch as cotton. The belts according to the invention woven from theseyarns are strong in the machine direction and sufficiently rigid in thecross machine direction.

[0017] Most preferred are aramid yarns due to their low weight and highstrength.

[0018] A non woven fabric substrate may be provided in place of a wovensubstrate and be formed from any one, or a blend or mixture of any ofthe above mentioned yarns or fibres. More than one nonwoven substratemay be provided, preferably two, and they may be vertically aligned oroffset relative to one another.

[0019] A belt substrate may comprise a non woven fabric with additionalspaced apart linear yarns extending substantially in a common direction,and a polymeric matrix material interconnecting and at least partiallyencapsulating each of the yarns. The linear yarns preferably areoriented in the running direction of the belt, but may also or insteadbe oriented in the cross-machine direction, i.e. transversely of thebelt e.g. as described in GB-A-2202873. Extra reinforcing yarnsextending substantially in the machine direction may also be provided.

[0020] The belt substrate preferably has a relatively high open area dueto the increase in delamination resistance, particularly if thesubstrate is fully impregnated with polymer. For this, a spiral linkbelt of the kind disclosed in GB-A-2051154, comprising an array of eg.cross-machine direction hinge wires, connected by interdigitatingflattened helical coils is particularly preferred, as one large openarea woven fabrics. This substrate may support a woven or non-wovenfabric which is coated or partially or fully impregnated with thesuitable polymer.

[0021] The surface of the belt may be formed with grooves extending inthe running direction of the belt to remove wet slurry generated duringthe polishing process. This slurry can be removed from the belt groovesusing one or more high pressure water jets, rotating fine brushes orhard non-metallic (e.g. ceramic) stylii.

DESCRIPTION OF DRAWINGS

[0022]FIG. 1 is a diagram of a continuous belt-type apparatus forpolishing silicon wafers, of the kind incorporating a belt in accordancewith the invention;

[0023]FIG. 2 is a fragmentary enlarged diagrammatic cross-section takenacross the machine direction of one embodiment of polishing belt of theinvention;

[0024]FIG. 3 is a view similar to FIG. 2 of another embodiment of thebelt of the invention;

[0025]FIG. 4 is a view similar to FIGS. 2 and 3 of yet anotherembodiment of the belt according to the invention;

[0026]FIG. 5 is a similar view of a fourth embodiment of the beltaccording to the invention;

[0027]FIG. 6 is a similar view of a fifth embodiment of a belt accordingto the invention;

[0028]FIG. 7 is a similar view of a sixth embodiment of belt accordingto the invention;

[0029]FIG. 8 is a similar view of a seventh embodiment of belt accordingto the invention; and

[0030]FIG. 9 is a similar view of an eighth embodiment of belt accordingto the invention;

DESCRIPTION OF EXEMPLIFIED EMBODIMENTS

[0031]FIG. 1 is a diagrammatic view of a continuous belt machine forpolishing and planarising silicon wafers. A platen 10 operable by ahydraulic or pneumatic ram 11, holds a silicon wafer 12 flat on thesurface of a continuous belt 13, after the wafer 12 has been put inplace by a remotely controlled or autonomous handling system such as arobotic arm (not shown). Belt 13 is passed around end rollers 14, 15 andis driven in the sense indicated by the arrows on the drawing. Apolishing slurry, containing very fine grade abrasive is fed onto theupper surface of the belt from a reservoir 16, through a feeder 17. Anexample of a suitable polishing slurry is disclosed in WO 96.16436 byAdvanced Micro Devices, Inc. The feeder 17 may be associated with meansknown in the prior art for achieving the desired distribution of theslurry on the belt, prior to encountering the wafer 12 which is to bepolished by the chemical-mechanical polishing process.

[0032] Polishing is achieved by the motion of the belt 13 in contactwith the surface of the wafer 12 which is to be polished, in forcedcontact under pressure with the wafer surface, from the platen 10 andram 11.

[0033] In accordance with the invention the belt 13 is made from asubstrate at least coated with a suitable polymeric material and somepossible structures are illustrated in the following figures by way ofexample.

[0034] In FIG. 2 a non-woven fibrous batt 20, preferably impregnated andreinforced with a suitable resin, is coated on its upper surface, forcontacting wafers to be polished, with a layer 21 of polyurethane havinga low Shore-A hardness. The upper surface is formed with a multitude ofparallel machine-direction grooves 22 for drainage of the used slurry(comprising abrasive particles, liquid medium and particles of siliconremoved from the wafer) from the polishing site.

[0035] In FIG. 3 a woven substrate 30 is shown, comprising machinedirection yarns 31, with cross-machine direction yarns 32 interwoventhrough them. The simplest possible weave pattern is shown, but ofcourse more complex weave patterns, including multi-tier MD yarns 31 maybe used, to obtain a bulkier woven substrate. Multiple layers of wovensubstrate 30 may be overlaid and impregnated with a binder or resin ifdesired. The yarns 32 may run in the cross-machine direction with theinterwoven yarns 31 extending in the machine direction. The substrate 30is coated on its upper polishing surface with a layer 33 of polyurethanehaving a low Shore-A hardness. This preferably strikes into the wovensubstrate, and may impregnate the substrate completely.

[0036] In FIG. 4 a non woven substrate 40 comprises an array of yarns41, extending eg in the machine direction, encapsulated in a polymericmaterial matrix 42. A coating 43 of a polyurethane having a low Shore-Ahardness is provided on the polishing surface of the substrate 40. Thesubstrate may be of a kind described in GB-A-2202873 and may includevertical passages through the substrate as disclosed in thatspecification.

[0037] In FIG. 5 a substrate 50 is provided which comprises a link beltof the kind disclosed in GB-A-2051154. This has an array ofcross-machine direction hinge-wires 51, each pair of which are connectedby receptive flattened helical coils 52, which each interdigitate withthe adjacent coils about the respective hinge wires. Substrate 50 iscovered with a fibrous layer, such as a non woven plastics impregnatedand reinforced batt 53, which is in turn coated with a layer 54 of a lowShore-A hardness polyurethane.

[0038] The hinge wires 51 and helical coils 52 may be of a suitablepolyamide material or less preferably of metal wire.

[0039]FIG. 6 illustrates another embodiment of belt which comprises asupporting substrate 60, and two layers of different hardness materials.These comprise an upper layer 61 of a relatively hard material, such asa polyurethane with 60-70 Shore-D hardness. Layer 61 provides an uppersurface 62 which is formed with parallel machine direction grooves 63for drainage of used slurry from the polishing site. A second,intermediate layer 64 is sandwiched between the relatively hard upperlayer 61, and the substrate 60 and comprises a relatively soft materialsuch as 60-70 Shore-A hardness polyurethane. The substrate 60 comprises,as in FIG. 2 a non-woven fibrous batt which is impregnated andreinforced with a suitable resin.

[0040] The structure superposing a relatively hard top surface materialover a relatively soft layer provides the benefits of a hard outersurface 62, with the resilience of the softer layer 64, reduces pressureon the wafer and thereby minimises the risk of wafer breakage.

[0041]FIG. 7 illustrates a further embodiment of belt which comprises awoven supporting substrate 70, carrying an upper layer 71 of arelatively soft material, such as 60-70 Shore-A hardness polyurethane,providing an upper surface 72 with drainage grooves 73, and anintermediate sandwiched layer 74 of a relatively hard material, such as60-70 Shore-D hardness polyurethane. This arrangement is essentially thereverse of that of FIG. 6, but gives a compliment top surface and hardmiddle layer which provides the stiffness necessary to hold the wafer inplace during planarisation.

[0042]FIG. 8 shows another embodiment of belt according to theinvention, comprising a supporting substrate 80 in the form of amembrane having machine direction reinforcing yarns 81 embedded therein.The membrane 80 may be perforated, although this is not shown. Membranesubstrate 80 carries an upper layer 82 of foamed plastics material, egpolyurethane. This foam may be rigid or preferably flexible, andprovides surface porosity to retain slurry material generated duringplanarisation. The necessary stiffness to hold the wafer in place isprovided by an intermediate layer of harder, eg 60-70 Shore-D hardnesspolyurethane 83.

[0043]FIG. 9 shows a yet further embodiment of belt comprising a spirallink fabric substrate 90, carrying an intermediate relatively hard layer91, of eg 60-70 Shore-D hardness polyurethane, carrying an upper layer92 of solid polyurethane containing beads which are heat activatedduring polyurethane curing to form pores in the surface, similar to afoam coating. The beads comprise expanded polystyrene pellets which aredispersed into the polyurethane.

[0044] The upper layer in any of the described embodiments may compriseat least one layer of partially fused polymeric particles, and/orcomprise two or more thermoplastic polymers having different meltingpoints. The sintered layer may optionally be reinforced by a textilematerial, e.g. a membrane, woven or nonwoven fabric, or chopped fibres.The layer may incorporate hollow microbeads of plastics, glass orsoluble material (such as CMC) which latter break down to provide aporous surface. Glass beads are used for their abrasive properties.

[0045] Abrasive particles or fibres, such as TiO₂; CeO₂; SiC; Si₃N₄;Al₂O₃; glass; silicates; BaCO₃; CaCO₃; diamond or carbon may be added tothe upper layer, which may also or instead consist of a transparentcoating.

[0046] The surface of the upper layer may be provided with a microtextured coating, that is with micro-scale grooves or roughness, formedfor example by machining, laser cutting (preferably with an ablation orexcimer laser), or chemical means (e.g. by dissolving soluble particlessuch as sugar or cooking salt present in the upper layer.

[0047] Upon curing of the polyurethane these pellets expand to formhollow beads which are cut open when the cured belt is conditioned eg bygrinding, providing location on the belt surface which can retainslurry.

[0048] Any of the various substrates illustrated may be used incombination with any of the single layer (FIGS. 2 to 4) or double layer(FIG. 5 to 9) structures described.

[0049] In the above embodiments the substrate fabric 20, 30 or coverlayer 53 may be an endless woven material to avoid the weakness importedby a splice or seam. The fabric may be woven from yarns of a hightensile strength and relatively low elongation, such as meta- or para-aramids, eg KEVLAR, NOMEX or TWARON; as well as PBO or its derivatives;polyetherimide, polyetherketone, PEEK, gel-spun UHMW polyethylene (egDYNEEMA or SPECTRA); or polybenzimidazole. Yarns of these compositionsmay be mixed or blended and mineral fibres such as glass, carbon orceramic yarns including rock fibres (eg basalt) on there own or mixed orblended with polymer yarns may be used. The aramids are most preferredhowever on account of their low weight and high strength.

[0050] The coating may also be any high abrasion resistance thermoset orthermoplastic polymer such as aliphatic polyamides, aliphatic aromaticcopolyamides, silicones or epoxy resins.

[0051] Woven metal mesh and perforate metal sheet belt substrate may beused with the belt interstices being occupied by rivets or fillers ofpolymeric material, improving bond strength between the polymer and themetal.

[0052] The main advantage of a chemical-mechanical polishing beltaccording to the invention is that improved bond strength is obtainedbetween the preferably synthetic polymer substrate and the polymercoating. As a result, not only does the coating tend not to flake off soreadily, but thicker coatings can be applied, possibly impregnating asubstantial proportion of the substrate or even fully encapsulating it,meaning that belts last a lot longer on the machines before needing tobe removed.

[0053] The belt is typically 1.5-3 metres in length, measured as theinner circumference of the endless belt, 0.2-0.6 metres in width, and0.1-0.6 cm thick. The coating typically comprises 40-70% of thethickness.

[0054] The belt according to the invention may be applicable in otherindustries, for example for polishing and planarising optical flats andmirrors prior to coating of the latter with a reflective metallic layer.

1. For use in polishing silicon wafers, an endless belt to act as apolishing tool, wherein said belt comprises a woven or non-woven fabriccoated with a suitable polymer.
 2. A belt according to claim 1, whereinsaid polymer is polyurethane, having a low Shore-D hardness.
 3. A beltaccording to claim 2, wherein the Shore-D hardness of the polyurethaneis in the range 65-75.
 4. A belt according to claim 1, wherein saidpolymer is a thermoset or thermoplastic polymer having a high abrasionresistance.
 5. A belt according to claim 4, wherein said polymer isselected from the group comprising: polyamides, silicones,fluropolymers, epoxy resins and thermoplastic polyurethanes.
 6. A beltaccording to claim 2, wherein the coating comprises an upper layer andan intermediate layer, of materials having different hardness.
 7. A beltaccording to claim 6, wherein the upper layer is of a harder materialthan the intermediate layer.
 8. A belt according to claim 6, wherein theupper layer is of a softer material than the intermediate layer.
 9. Abelt according to claim 8, wherein the upper layer is of a foamedplastic material.
 10. A belt according to claim 8, wherein the upperlayer comprises a layer of beads of plastic, glass or soluble material.11. A belt according to claim 10, wherein said beads comprise expandedpolystyrene pellets which are dispersed into the upper layer.
 12. A beltaccording to claim 6, wherein abrasive particles or fibres areincorporated in the upper layer.
 13. A belt according to claim 6,wherein the surface of the upper layer is provides with a micro-texturedcoating.
 14. A belt according to claim 1, wherein said fabric is wovenin endless form and embodies yarns of high tensile strength and lowelongation.
 15. A belt according to claim 6, wherein said high tensilestrength yarns are selected from the group comprising:—meta orpara-aramids; polyetherimide; polyimide; polyetherketone; PEEK; gel spunUHMW polyethylene; and polybenzimidazole.
 16. A belt according to claim6, wherein said high tensile strength yarns comprise a mixture or blendof two or more such yarns.
 17. A belt according to claim 6, wherein saidyarns are of any one of:—glass fibres, carbon or ceramic yarns, basaltfibres, other rock fibres or mixtures of mineral fibres with syntheticpolymer yarns.
 18. A belt according to claim 1, wherein said fabric is anon woven fabric formed from one or more yarn staples.
 19. A beltaccording to claim 18, wherein said yarn staple comprises one or moregroups of fibres selected from the group comprising:—meta or paraaramids; polyetherimide; polyimide; polyetherketone; PEEK; gel-spun UHMWpolyethylene; polybenzimidazole; glass fibres, carbon fibres, ceramicfibres; basalt fibre; other rock fibres.
 20. A belt according to claim1, wherein said fabric comprises a non woven fabric incorporatingadditional spaced apart linear yarns extending substantially in a commondirection and a polymeric matrix material interconnecting and at leastpartially encapsulating each said yarn.
 21. A belt according to claim20, wherein said linear yarns are oriented in the running direction ofsaid belt.
 22. A belt according to claim 20, wherein said linear yarnsare oriented in transversely of said belt.
 23. A belt according to claim22, wherein additional reinforcing yarns are provided extending in therunning direction of said belt.
 24. A belt according to claim 1, whereinsaid belt has a relatively high open area.
 25. A belt according to claim24, wherein said belt includes a spiral-link belt.
 26. A belt accordingto claim 25, wherein said spiral-link belt supports a woven or non wovenfabric layer which is coated or impregnated with said polymer.
 27. Abelt according to claim 1, wherein the surface of said belt is formedwith grooves extending in the running direction of the belt to removewet slurry generated in use.
 28. Apparatus for polishing silicon wafers,incorporating an endless belt to act as a polishing tool, wherein saidbelt is a belt according to claim 1.